Dengue virus infections are a significant cause of morbidity and mortality in many areas of the world, and dengue has recently been reported in southern U.S. states. Epidemiological studies have shown that the severe complications of infections; dengue hemorrhagic fever (DHF)/dengue shock syndrome (DSS) are much more commonly observed during secondary infections with a different serotype of dengue virus from that which caused the primary infection, with a different serotype of dengue virus from that which caused the primary infection, and it has been assumed that DHF/DSS are caused by immunopathological mechanisms. Safe and effective dengue vaccines are an important public health need. Vaccines should induce protective immune responses, but should not induce immune responses which may cause DHF/DSS. The aim of this project is to define the protective immune responses in dengue virus infections and the pathogenesis of DHF/DSS. No animal models are available to study DHF/DSS; therefore, research using human subjects is required to provide information essential for vaccine development. In this research program we will; 1) analyse dengue virus-specific T cell responses in bulk culture and at the clonal levels, 2) establish dengue virus-specific cytotoxic T lymphocyte (CTL) clones and helper T (Th) cell clones, 3) define the specificity, HLA- restrictions and functions (cytotoxic activity, helper activity, lymphokine production, etc.) of these T cell clones. Furthermore, we will 4) map the epitopes recognized by T cell clones, using recombinant viruses, purified proteins and synthetic peptides. Monocytes are the most permissive human cell for growth of dengue virus. We will analyse the effect of lymphokines produced by dengue virus-specific T cells on dengue virus infection of monocytes with/without enhancing antibodies, and characterize the lysis of dengue infected monocytes by CTL. We will evaluate the in vivo activation of T lymphocytes and monocytes during dengue infections, DHF and DSS, and define the role of lymphokines, monokines and CTL in recovery from infection and in the pathogenesis of DHF/DSS. We will correlate the levels of cytokines and CTL response with the severity of symptoms and the outcome of dengue virus infection. We will measure the levels of cytokines and CTL in volunteers immunized with experimental live vaccines.